Improving fuel economy is a continuing goal sought to be reached by internal combustion engine designers and manufacturers. Fuel injectors and their associated components have long been the focus of design improvements to achieve this objective. In addition, the elimination or reduction of injector carboning and corrosion are problems indirectly affecting fuel economy also sought to be solved. Advancing injection timing at higher loads will additionally result in improved engine performance and fuel economy and, consequently, is also a goal of engine designers.
Compression ignition or diesel engine manufacturers have long sought a reliable and consistent means for altering injection timing to enhance fuel economy. The injectors in a typical diesel engine are operated by a camshaft with a plurality of precisely defined lobe profiles positioned radially in a timed rotational relationship. Each lobe is connected to a camshaft-operated injector by a mechanical linkage which produces a rigid timing program that is not easily altered. Variable length hydraulic tappets positioned between the camshaft and injectors alter the engine timing by selectively lengthening the timing drive train, which changes the effective profile of the camshaft. When a hydraulic tappet is collapsed or shortened, the injector functions in its normal timing sequence. When the tappet is lengthened by trapping hydraulic fluid in an internal tappet chamber, the drive train between the camshaft and injector is lengthened which advances the normal timing sequence. If it is desired to retard timing, the tappet can be selectively collapsed to shorten the drive train. However, the use of variable length hydraulic tappets to advance injection timing has been found to be unsuitable for highly loaded unit fuel injectors because of a variety of problems, including that of secondary injection which leads to increased particulate levels.
A pressure limiting hydraulic tappet useful with a highly loaded fuel injector to advance timing is disclosed in U.S. Pat. No. 4,395,979 to Perr. This hydraulic tappet selectively varies the effective profile of the camshaft by extending the drive train between the camshaft and injector and contracting when the drive train pressure reaches a predetermined maximum. This tappet is pressure limiting and, therefore, does not collapse when the predetermined pressure is reached, thus forming a load cell. Although this hydraulic tappet allows better control over injection timing than tappets without a pressure limiting feature, the load is not maintained at a constant level during blow down, and it operates at lower loads than necessary to optimize injection timing.
Another type of pressure limiting hydraulic tappet is described in U.S. Pat. No. 4,407,241 to Butler et al. This tappet includes an expandable piston which defines a load cell chamber with selectively opened inlet and exit valves. The exit valve has one or more exit flow rates which are altered in response to the load cell chamber pressure. The closing of the exit valve is eased by dampening means. This tappet arrangement also provides better control over injection timing than many designs; however, the blow down pressure of the load cell cannot be increased to advance injection tinting to improve fuel economy.
U.S. Pat. No. 4,254,749 to Krieg et al discloses a device for hydraulically altering fuel injection timing in response to a high pressure fluid signal. This device does not include dampening structure, however, and thus does not control the impact load on the plunger or allow blow down pressure to be increased. As a result, optimum fuel economy is not likely to be attained with this structure.
The prior art, therefore, has failed to provide a dampened or pressure regulating load cell injector tappet which allows the blow down pressure of the load cell to be increased, thus optimally advancing injection timing at higher loads for improved fuel economy. A need for such an injector tappet exists to optimize injection timing and improve fuel economy.